1. Field of the Invention
The invention relates to a piston pump intended in particular for pumping brake fluid in a vehicle brake system that has traction control and/or an electrohydraulic vehicle brake system.
2. Description of the Prior Art
The known piston pumps of the type with which this invention is concerned have a pump piston, which is received axially displaceably in a pump bore that is mounted in a pump housing. For driving the pump piston, the known piston pumps have a rotationally drivable eccentric element, which is disposed on one face end of the pump piston in the pump housing. By rotational drive of the eccentric element, the pump piston is driven to execute an axially reciprocating stroke motion in the pump bore and in so doing pumps fluid in a manner known per se. For sealing off the pump piston in the pump bore, the known piston pumps have a sealing ring, which is disposed between a pump inlet and the eccentric element in the pump bore. This sealing ring seals off the piston pump from the eccentric element. When brake fluid is aspirated, this sealing ring prevents the aspiration of air, for instance, into the pump bore from the side of the eccentric element. The sealing ring also prevents the fluid that is to be pumped from escaping out of the pump bore in the direction of the eccentric element.
In the piston pump of the invention having the characteristics of claim 1, the sealing ring, which seals off the piston pump from the eccentric element, has two axially spaced-apart, encompassing sealing points. The fact that the pump piston or pump bore is sealed off at two axially spaced-apart points already provides an improvement in the sealing action. Especially in the event of a temporary or even a permanent leak of a sealing point, the piston pump is still sealed off from the eccentric element by the other sealing point of the sealing ring. The piston pump remains capable of pumping without restriction. A temporary leak can be caused for instance by dirt particles, which reach the sealing point between the sealing ring and a surface of the pump piston or pump bore on which the sealing ring rests with the encompassing sealing point. In the low-temperature range, for instance of xe2x88x9230xc2x0 and below, as well, a sealing action of the sealing points of the sealing ring can be impaired. If the temperature rises to higher values again, the sealing action is again fully available.
The piston pump of the invention also has a fluid line, which connects an interstice between the two sealing points of the sealing ring to a supply of fluid to be pumped by the piston pump. The fluid line connects the interstice between the two sealing points of the sealing ring in particular to a supply container that contains the fluid to be pumped, or to a pump inlet. Through the fluid line, the piston pump of the invention, in the event of a temporary or permanent leak of one sealing point of the sealing ring, aspirates fluid out of the supply into the pump bore. This prevents air, for instance, from being aspirated from the eccentric element into the pump bore in the event of a temporary or permanent leak of the sealing ring. If fluid under pressure reaches the sealing ring during a pumping stroke of the piston pump, and if the sealing ring has a temporary or permanent leak, then the fluid passes through the fluid line into the supply of fluid to be pumped and does not escape out of the pump bore in the direction of the eccentric element. The invention has the advantage that in the event of a temporary or permanent leak of the sealing ring, an aspiration of air, for instance, from the direction of the eccentric element into the pump bore is avoided, as is an escape of fluid in the direction of the eccentric element during a pumping stroke. Another advantage of the invention is that with slight changes, it can be realized in existing piston pumps; an existing piston pump need not be completely reconstructed or substantially altered in order to realize the invention; slight changes that can be accomplished at little effort or expense make it possible to realize the invention in existing piston pumps. Especially in large-scale mass production of a piston pump, the invention can be adopted at little effort or expense.
The piston pump of the invention is intended in particular as a pump in a brake system of a vehicle and is used to control the pressure in wheel brake cylinders. Depending on the type of brake system, the abbreviations ABS (for anti-lock brake system), TCS (traction control system), VDC (vehicle dynamics control) and EHB (electrohydraulic brake system) are used for such brake systems. In the brake system, the pump serves for instance to return brake fluid from a wheel brake cylinder or a plurality of wheel brake cylinders to a master cylinder (ABS) and/or to pump brake fluid out of a supply container into one or more wheel brake cylinder (TCS or VDC or EHB). In a brake system with wheel slip control (ABS or TCS) and/or a brake system serving as a steering aid (VDC) and/or an electrohydraulic brake system (EHB), the pump is needed. With wheel slip control (ABS or TCS), locking of the wheels of the vehicle during a braking event involving strong pressure on the brake pedal (ABS) and/or spinning of the driven wheels of the vehicle in the event of strong pressure on the gas pedal (TCS) can for instance be prevented. In a brake system serving as a steering aid (VDC), a brake pressure is built up in one or more wheel brake cylinder independently of an actuation of the brake pedal or gas pedal, for instance to prevent the vehicle from breaking out of the track desired by the driver. The pump can also be used in an electrohydraulic brake system (EHB), in which the pump pumps the brake fluid into the wheel brake cylinder or cylinders if an electric brake pedal sensor detects an actuation of the brake pedal, or in which the pump is used to fill a reservoir of the brake system.